1. Field of the Invention
The present invention relates generally to the separation of isotopes by a thermally driven exchange process which utilizes two flow paths that are interconnectable within a system and, more particularly, to such a process which includes a single mass spectrometer operable to switch between flow paths in order to enhance the isotopic separation.
2. Background Information
Natural boric acid solutions containing a Boron 10 (B-10) to Boron 11 (B-11) atomic ratio of 19.8:80.2 are used as control fluids in nuclear power plants. It is known that the B-10 isotope is responsible for nuclear reaction control due to its neutron capturing ability. B-10 enriched boric acid solutions which contain a B-10 to B-11 atomic ratio in excess of 19.8:80.2 are not currently employed in reactor coolant systems since the B-10 enriched solutions may cost as much as $3.00 (U.S.) per gram of B-10 while the reactor grade natural boric acid solution may only cost $1.00 (U.S.) per pound of boron. Therefore, it would be most advantageous to be able to enrich boron-containing products in their B-10 content using an inexpensive process.
A number of methods are known for increasing the B-10 content of common boron compounds, such as by physical or chemical processes or a combination of these two processes. Such methods include distilling, solvent extracting, and ion exchanging the boron compounds.
A thermally driven ion exchange apparatus and method for producing enriched B-10 are described in detail in U.S. Pat. No. 5,176,885 and assigned to the assignee of the present invention. This application teaches a method and apparatus for enriching B-10 content by flowing a boron solution through two interconnecting flow paths having equal flow rates. This system comprises a hot tank and a cold tank, a heater and a cooler, and two resin-containing ion exchangers A and B. The following is a description of the process beginning with the cold tank.
In the flow path originating with the cold tank, boron solution flows out of the cold tank into a heater and thereafter into ion exchanger B. The heated boric acid solution initially preferentially desorbs B-10 when passed through the resin in ion exchanger B. Next, the heated boric acid solution somewhat enriched in B-10 by interaction with the resin in ion exchanger B flows into the hot tank. Simultaneously with the flow initiation of the cold tank loop, the boric acid solution in the hot tank flows into the cooler and thereafter into ion exchanger A. The B-10 in the cooled boric acid solution will initially be preferentially stored on the resin in ion exchanger A. This solution somewhat depleted of boron and particularly of boron-10 by interaction with the resin in ion exchanger A then flows into the cold tank. The above process is continued until a predetermined switchover point occurs and then system valve positions are switched to allow the heated boric acid solution to pass through ion exchanger A and the cooled boric acid solution to pass through ion exchanger B. This process of periodically switching the boric acid solution flow paths is continuously repeated until further cycling will not contribute significantly to B-10 enrichment. The final result is enriched boron in both the hot and cold tanks.
To appreciate the point at which switchover should occur, the relationship between ion exchangers A and B and the hot and cold tanks should be understood. As previously stated, heated boric acid solution when passed through either ion exchanger A or B will initially preferentially elute B-10 containing ions, thus increasing the isotopic ratio the solution passing into the hot tank. Cooled boric acid solution will initially preferentially store B-10 containing iols on the resin in either ion exchanger A or B, thus somewhat decreasing the isotopic ratio of B-10 to B-11 in the solution passing into the cold tank. As time passes, the isotopic ratios of B-10 to B-11 in the ion changers begins to reverse the above-stated trend, such as the ratio of B-10 to B-11 decreasing in the hot ion exchanger and increasing in the cold heat exchanger. This is because the preferential transfer of B-10 from or to the resin decreases resulting in the isotopic ratio of B-10 to B-11 in the hot solution exiting the hot ion exchanger gradually decreasing and the cold solution at the exit of cold ion exchanger is gradually increasing. Switchover should occur when the isotopic ratio of B-10 to B-11 in the hot solution exiting the hot ion exchanger has decreased to a point where it is substantially identical to that in the hot tank at that time.
The switchover point in the copending application referred to above is determined by using two boron isotopic analyzers, such as mass spectrometers. One of the analyzers is connected alternately to ion exchanger A and the hot tank and the other analyzer is connected alternately to ion exchanger B and the hot tank. Due to specific process requirements, only one analyzer will be active at any given time period (the analyzer measuring the heated solution). If, for example, the analyzer connected to ion exchanger A is active, that analyzer will alternately determine the B-10:B-11 ratio at the exit of ion exchanger A and in the hot tank. If the analyzer connected to ion exchanger B is active, that analyzer will determine the B-10:B-11 ratio at the exit of ion exchanger B and in the hot tank.
In this arrangement, the active or on-line analyzer determines the boron content from ion exchanger A or B and then immediately switches to determine the boron content in the hot tank. Since two analyzers are required in this two loop system, they need to be calibrated with each other to ensure accurate and consistent switchover occurs. Obviously, this calibration requirement increases system maintenance downtime and operating costs.
Therefore, what is needed is an improved instrumentation and control scheme which enhances the operation of the two loop boron isotopic enrichment system.